Bi-directional selector switch with slidable pawl transverse to plunger axis and ratchet wheel plane

ABSTRACT

A bi-directional rotary ratchet mechanism and rotary stepping switch embodying the mechanism. The ratchet mechanism has a pair of actuating plungers straddling a ratchet wheel and each rockably mounting a spring-loaded wheel actuator or pawl which rotates under spring action into driving engagement with the ratchet wheel to step the latter in one direction of rotation upon movement of the respective plunger through a wheel driving stroke. During the return stroke of the plunger to its normal position, the pawl engages an interior surface of the ratchet housing which rotates the pawl outwardly away from the ratchet wheel against spring action to a position where the pawl clears the wheel for stepping rotation in the opposite direction by the other plunger and pawl. The rotary stepping switch has electrical contacts mounted on the ratchet wheel and housing so that stepping rotation of the wheel through its stepping positions results in successive closure of the switch contacts.

United States Patent Jordan et al.

[54] III-DIRECTIONAL SELECTOR SWITCH WITH SLIDABLE PAWL TRANSVERSE TO PLUNGER AXIS AND RATCHET WHEEL PLANE [72] Inventors: Lewis W. Jordan, Newhall; Leon A. Cerniway, Burbank, both of Calif. [73] Assignee: Janco Corporation, Burbank, Calif. [22] Filed: Feb. 12, 1970 [21] Appl.No.: 10,738

[151 3,654,413 [451 Apr. 4, 1972 FOREIGN PATENTS OR APPLICATIONS 227,357 3/1960 Australia ......200/l56 Primary Examiner-Robert K. Schaefer Assistant Examiner-Robert A. Vanderhye Attorney-Forrest J. Lilly [57] ABSTRACT A bi-directional rotary ratchet mechanism and rotary stepping switch embodying the mechanism. The ratchet mechanism has a pair of actuating plungers straddling a ratchet wheel and each rockably mounting a spring-loaded wheel actuator or pawl which rotates under spring action into driving engagement with the ratchet wheel to step the latter in one direction of rotation upon movement of the respective plunger through a wheel driving stroke. During the return stroke of the plunger to its normal position, the pawl engages an interior surface of the ratchet housing which rotates the pawl outwardly away from the ratchet wheel against spring action to a position where the pawl clears the wheel for stepping rotation in the opposite direction by the other plunger and pawl. The rotary stepping switch has electrical contacts mounted on the ratchet wheel and housing so that stepping rotation of the wheel through its stepping positions results in successive closure of the switch contacts.

11 Claims, 8 Drawing Figures PATENTEDAPR 41912 SHEET 2 [IF 2 INVENTOR.

FIELD OF THE INVENTION This invention relates generally to ratcheting devices and more particularly to a rotary ratchet mechanism and to a rotary stepping switch embodying the ratchet mechanism.

BACKGROUND OF THE INVENTION In its broader aspects, the invention provides a rotary ratchet mechanism which possesses the advantages of simplicity of construction, low production cost, and reliability of operation, and which is capable of varied useful applications.

A more limited aspect of the invention is concerned with a ro- 1 tary electrical stepping switch embodying the ratchet mechanism.

Generally speaking, the broader aspect of the invention relates to a ratchet mechanism of the class having a rotary ratchet wheel and a pair of ratchet wheel actuators including pawls mounted on reciprocating carriers which are movable through a wheel driving stroke and a return stroke. Movement of one actuator through alternate driving and return strokes steps the ratchet wheel in one direction through a number of successive positions. Movement of the other actuator through alternate driving and return strokes steps the ratchet wheel in the opposite direction through its successive positions.

Ratchet mechanisms of this class may be used for various purposes. By way of example, it is known to use such a ratchet mechanism in a rotary electrical switch. In this case, the ratchet wheel provides a rotary electrical contact having a number of switching positions corresponding to the successive stepping positions of the ratchet wheel. Thus, the switch may be selectively actuated to step the rotary electrical contact of the switch through its successive stepping positions in either direction of rotation. A typical state of the art switch of this kind is disclosed in the Phlager U.S. Pat. No. 3,435,167.

The existing ratchet and switch devices of the class described possess certain inherent deficiencies which this invention overcomes. Foremost among these deficiencies are complexity of construction, high production cost, and low operating reliability. Such deficiencies result in large part from the fact that the ratchet wheel actuator pawls are integrally joined to their respective reciprocating carriers in such a way that the pawls bend or flex relative to the carriers into and from driving engagement with the ratchet wheel. In the above mentioned Phlager patent, for example, the reciprocating carriers are plates and the pawls are resilient arms on the carrier plates formed by slots in the plates. These carrier plates are also slotted to defined resilient detent arms which serve as indexing means to locate the ratchet wheel in its successive stepping positions and prevent retrograde rotation of the wheel during the return stroke of each carrier plate and its respective wheel actuator.

The disadvantage of this type of ratchet structure resides in the fact that the ratchet wheel actuators must be made and located relative to the ratchet wheel with great accuracy to as sure proper ratcheting engagement of the actuator pawls with the wheel. Alternatively, other means, such as the complex camming arrangement of Phlager, must be provided to move the pawls into and from engagement with the ratchet wheel. The end result of this type of ratchet construction is excessive complexity and size, high production cost, and low operating reliability.

SUMMARY OF THE INVENTION The present invention overcomes the above and other drawbacks of the existing ratchet and ratchet switch devices of the character described. To this end, the invention provides a ratchet mechanism wherein the carrier of each ratchet wheel actuator comprises a plunger, and each actuator pawl is separately formed from and rockably supported in a unique manner on its respective plunger. A spring urges each actuator pawl to extended position relative to its plunger, wherein the pawl is disposed to step the ratchet wheel from one position to the next during successive driving strokes of the plunger. Each spring also urges the corresponding plunger through its return stroke to a normal position, wherein the actuator is conditioned for its next ratchet wheel driving stroke.

One of the major advantages of the invention resides in the fact that each ratchet wheel actuator pawl is movable to and from engagement with the ratchet wheel with a pivoting or rocking motion, rather than with a bending or flexing motion as in the prior art devices. The present rocking motion of the pawls provides the latter with much greater freedom of positioning movement relative to their respective carriers or plungers than possessed by the existing flexible pawls. This greater freedom, in turn, permits the present ratchet mechanism to be manufactured and assembled without the necessity of making and locating the parts with the high degree of accuracy required in the existing ratchet mechanisms. The construction of the present ratchet mechanism is thus simplified, its production cost is reduced, and its operating reliability is greatly enhanced. Moreover, it will appear from the ensuing description that the present ratchet mechanism may be made in much smaller size than the existing ratchet devices of the same class.

The present rotary stepping switch uses the ratchet wheel of the present ratchet mechanism as a rotary switch wheel or contact device. Associated with this rotary'contact are stationary contacts on the ratchet housing, preferably furnished by a printed circuit, which are engaged in succession by the rotary contact as it steps from one position to the next. The disclosed embodiment of the invention is a stepping switch whose housing has a front wall through which the ratchet plungers extend to form push buttons for stepping the switch wheel in either direction of rotation. A window in this front wall exposes indicia, such as numbers, about the perimeter of the wheel for indicating its current position.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation of a rotary stepping switch according to the invention;

FIG. 2 is a side elevation of the switch;

FIG. 3 is an enlarged section taken on line 3-3 in FIG. 1;

FIG. 4 is a section taken on line 4-4 in FIG. 3;

FIG. 5 is a fragmentary section illustrating one switch actuating plunger partially extended to a position corresponding to the start of its wheel engagement action;

FIG. 6 is a view similar to FIG. 5 illustrating the switch actuating plunger in its fully extended position;

FIG. 7 is a view similar to FIG. 6 illustrating the manner in which the plunger pawl moves over the ratchet wheel teeth during the return stroke of the plunger to its extended position; and

FIG. 8 illustrates a printed circuit embodied in the switch.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to'the drawings, there is illustrated a ratchet mechanism 10 according to the invention, embodied within a rotary switch 12 of the invention. The ratchet mechanism 10 has a support 14 which, in this instance, forms the case or housing of the switch 12. The support or housing may be conveniently injection molded from plastic and includes a side wall 16 bounded about its perimeter by a rear wall 18, top and bottom walls 20,22 and the front wall 24. The side of the housing opposite the side wall 16 is closed by a removable wall member 25. The housing and wall member are positively located relative to one another by small locating pins 26 in the housing which projects into sockets 28 in the wall member. The wall member 25 may be secured to the housing 14in any convenient manner. As shown, the side edge of the front housing wall 24 projects beyond the adjacent edges of the housing walls 18, 20, 22, and is recessed to receive the front edge of the wall member 25 in such a way that this member is flush with the edge of the front wall. The ends of the front housing wall 24 project beyond the top and bottom housing walls 20 and 22, and are apertured to form mounting flanges 32.

The present ratchet mechanism 10 is enclosed within the housing 14. The ratchet mechanism includes a ratchet wheel 34 rotatably supported on a journal 36 projecting from the inner side of the housing side wall 16. The ratchet wheel has a generally hollow configuration and includes a circular side wall 38 bounded by an annular rim wall 40. This rim wall projects axially beyond one side of the side wall to form a hollow wheel cavity open at one side of the wheel. This wheel is mounted on the housing journal 36 with the open side of the wheel facing the housing side wall 16. Integrally formed on the inner surface of the wheel side wall 38, concentric with the wheel, is a peripherally toothed ratchet hub 42 and a toothed wheel indexing hub 44. The ratchet hub 42 has ratchet teeth 46 uniformly spaced about its circumference. The indexing hub 44 has indexing recesses 48 uniformly spaced about its circumference. The ratchet wheel, like the ratchet housing, can be conveniently injection molded from plastic.

In addition to the ratchet wheel 34, the ratchet mechanism 10 includes a pair of ratchet wheel actuators 49 including plungers 50 which project slidably through bores 52 in the housing front wall 24 and through bores 54 in integral internal bosses 56 on the housing top and bottom walls 20,22. The plungers 50 parallel one another and are located at diametrically opposite sides of the ratchet wheel 34. The front ends of the plungers project beyond the front housing wall 24 and mount push buttons 58 for actuating the plungers in the manner hereinafter explained. About the front ends of the housing front wall bores 52 are bosses 60 with bearing inserts 61 to provide adequate bearing support for the plungers.

Each plunger 50 has a cylindrical front extremity 62 and a cylindrical rear extremity 64. The rear extremity is reduced in diameter relative to the front extremity to define an intervening rearwardly presented thrust shoulder 66 at the juncture of the two extremities-The front extremity 62 of each plunger 50 fits slidably within the corresponding front housing bore 52. The rear extremity of each plunger fits slidably within the corresponding housing boss bore 54. On the rear end of each plunger is a snap ring 68 which forms a stop shoulder engageable with the respective housing boss 56 to limit forward movement of the plunger relative to the housing 14.

Rockably mounted on the rear extremity 64 of each plunger 50 is a pawl 70. This pawl includes an arm 71 having its ends turned at right angles to form a mounting flange 72 and a drive flange 74. The pawl mounting flange 72 is apertured to fit rockably on the plunger extremity 64, rearwardly of the plunger shoulder 66. Surrounding the rear extremity 64 of each plunger is a coil spring 76. This spring seats at one end against the pawl mounting flange 72 and at its other end against the adjacent housing boss 56. Springs 76 serve two purposes. First, the springs urge the pawl mounting flanges 72 flat against the plunger shoulders 66, when the plungers 50 are depressed as in FIGS. and 6, to position the pawls both longitudinally and angularly relative to the plungers for driving engagement with ratchet wheel 34. Secondly, the springs urge the plungers 50 to their normal positions of FIG. 3.

The housing 14, ratchet wheel 34, plungers 50, and ratchet pawls 70 are constructed and arranged in such a way that the actuator pawl arms 71 extend between the housing side wall 16 and the ratchet wheel, in slidable seating contact with the wall. The drive flange 74 on the outer end of each pawl arm projects through the confronting open side of the ratchet wheel 34 to engage the ratchet hub 42 of the wheel, as explained presently.

Pivotally mounted on the housing side wall 16, between the plungers 50 and adjacent the rear housing wall 18, is an indexing member 78. This indexing member has an arm 80 which extends between the side wall 16 and ratchet wheel 34 in slidable seating contact with the wall. On the outer end of the indexing arm 80 is a detent 82 which projects through the open side of the ratchet wheel 34 to engage the indexing hub 44 of the wheel. Acting between the housing 14 and the indexing member 78 is a spring 84 which urges the indexing detent 82 against the indexing hub. As the ratchet wheel 34 turns, the spring-loaded detent 82 engages within the successive indexing hub recesses 48 to yieldably locate or index the wheel in predetermined angular positions, or index positions.

From the description to this point it will be understood that the actuator plungers 50 are yieldably urged by the springs 76 to normal forward limiting positions relative to the housing 14. When the plungers are depressed as in FIGS. 5 and 6, the actuator pawls 70 are yieldably urged by springs 76 to normal positions relative to the plungers, wherein the pawl mounting flanges 72 seat flat against the plunger shoulders 66. These latter positions of the pawls relative to the plungers are hereafter referred to as their extended positions. The pawls 70 are so shaped and dimensioned that in these extended positions, the drive flange 74 on each pawl is disposed to engage a tooth 46 of the ratchet wheel hub 42 when the corresponding plunger 50 is driven rearwardly through a ratchet wheel driving stroke relative to the housing 14 by finger pressure on its front button 58. Engagement of the rearwardly moving pawl drive flange 74 with the ratchet wheel tooth 46 rotates or steps the ratchet wheel 34 from its current index position to the-next position. As the wheel approaches its new position, the indexing detent 82 enters the corresponding indexing recess 48 in the indexing hub 44 to accurately index the wheel in its new position.

The depressed actuator plunger 50 is released at the end of its rearward ratchet wheel driving stroke to permit forward return of the plunger to its normal position under the action of the then compressed plunger spring 76. During this plunger return stroke, the actuator pawl drive flange 74 engages the following ratchet tooth 46 on the ratchet wheel 34 and is cammed outwardly away from the ratchet hub 46 by the tooth, against the action of the plunger spring 76, to permit the flange to pass over the tooth, as shown in FIG. 7. The plunger spring then returns the pawl flange to its normal position as shown in FIG. 3. It will be understood that the above description applies with equal force to both ratchet wheel actuators 49.

It will now be understood that actuation of one plunger 50 through successive driving and return strokes steps the ratchet wheel 34 through its successive index positions in one direction of rotation. Similar actuation of the other plunger steps the ratchet wheel through its successive H0lh 13/58 steps in the opposite direction of rotation. The indexing member 78 indexes the ratchet wheel 34 in its successive positions and, in addition, yieldably retains the ratchet wheel against retrograde rotation during the return stroke of each plunger. Thus, the ratchet wheel 34 can be selectively stepped through successive positions in either direction of rotation.

Since the ratchet wheel 34 rotates in both directions of rotation, it is necessary that the driving flanges 74 of the actuator pawls 70 be retracted clear of the circular path of the ratchet wheel teeth 46 when the plungers 50 occupy their normal forward limiting positions. This is necessary, of course, to prevent either actuator 49 from interfering with rotation of the ratchet wheel 34 by the opposite actuator in response to depression of its respective plunger 50. Obviously, this may be accomplished simply by extending the strokes of the actuator plungers 50 sufficiently in the forward direction to locate the actuator pawl drive flanges 74 clear of the ratchet hub 42 when the plungers occupy their forward limiting positions. However, this approach is undesirable since it increases the overall size and weight of the ratchet mechanism. According to a feature of the present invention, retraction of the drive pawls 70 clear of the ratchet wheel hub 42 when plungers 50 are in their forward limiting positions is accomplished by rearwardly presented pawl seating surfaces 86 on the inside of the front wall 24 of the ratchet housing 14, about the plungers. Each seating surface 86 is inclined at an oblique angle relative to the axis of the corresponding plunger. As each plunger 50 approaches its normal forward limiting position under the action of its return spring 76, the mounting flange 72 of its respective pawl 70 engages and rotates into flat face-to-face contact with the corresponding housing surface 86. This action rotates the pawl outwardly from its extended position adjacent the ratchet wheel to retract its drive flange 74 clear of the ratchet wheel teeth 46. Rearward depression of either plunger through its ratchet wheel driving stroke retracts the mounting flange 72 of its pawl 70 rearwardly from seating contact with the housing surface 86. The pawl is then returned by its spring 76 to its extended position relative to the plunger. This relocates the pawl drive flange 74 in proper position to drive the ratchet wheel 34 during the remaining portion of the plunger driving stroke.

Projecting from the housing side wall 16, within the region between the front housing wall 24 and the positions occupied by the pawl driving flanges 74 when the plungers 50 are in their forward limiting positions, are a pair of anti-jamming pins 88. These pins prevent jamming of the ratchet mechanism in the event both actuator plungers 50 are depressed simultaneously. Thus, if both plungers are depressed simultaneously, both actuator pawls 70 are extended into driving engagement with the ratchet wheel 34 and exert opposing forces on the wheel. If, under this condition, the plunger forces are not equal, the opposing couples on the ratchet wheel will tend to rotate the ratchet wheel, thereby tending to rotate the pawl to a wedging or jamming position between the plunger and wheel, thus jamming the mechanism. Accordingly, if sufficient pressure is applied to the plungers 50, one actuator may assume a jamming position, unless means are provided to prevent this occurrence. The anti-jamming pins 88 provide such means. Thus, these pins are located to act as stops which prevent the actuators from rotating to jamming positions between the ratchet wheel and the plungers in the event both plungers are depressed simultaneously.

It will be recalled that the illustrated embodiment of the invention is an electrical rotary stepping switch. In this embodiment, the ratchet wheel 34 serves as a rotary switch wheel which mounts electrical contact brushes 90. The removable side wall 25 of the housing 14 has a printed circuit 92 providing contacts engageable by the contact brushes 90. The printed circuit has externally accessible terminals 94 for connection to electrical leads. As the switch wheel 34 is stepped from one position to the next by depression of the actuator plungers 50, the wheel contact brushes 90 engage different contacts of the printed circuit 92 to complete electrical circuits between the switch terminals 94 in predetermined sequence. About the perimeter of the switch wheel 34 are indicia, in this instance numerals 96, which are exposed in succession through a window 98 in the front housing wall 24 to indicate the current position of the wheel.

We claim:

1. A ratchet mechanism comprising:

a support;

a hollow ratchet wheel rotatably mounted on said support and including a side wall, a coaxial hub projecting axially from one side of said side wall at the center of the wall and having ratchet teeth about its circumference, a coaxial cylindrical rim wall projecting axially from said one side of said side wall along the perimeter of the side wall and in surrounding relation to said hub, and said rim wall being radially spaced from and having at least substantially the same axial extent as said hub to define with the hub an annular cavity opening axially through the side of the ratchet wheel opposite said side wall;

a plunger mounted on said support for longitudinal movement along an axis parallel to the plane of the wheel between two limiting positions;

means for moving said plunger between said limiting positions; and

a ratchet pawl carried by said plunger and projecting from said plunger into said ratchet wheel cavity through the open side of the cavity in a direction transverse to both said plunger and plane for driving engagement with said ratchet teeth to step the ratchet in one direction through successive positions upon successive strokes of said plunger in one direction.

2. A ratchet mechanism according to claim 1, wherein:

said plunger is located in the plane of said ratchet wheel and said pawl projects from said plunger along said opposite wheel side and then into said wheel cavity.

3. A ratchet mechanism according to claim 2. wherein:

said pawl has a mounting flange with an opening loosely receiving said plunger to rockably support the pawl on the plunger for rocking motion between a retracted position wherein the pawl is disposed to clear said ratchet teeth and an extended position wherein the pawl is disposed to engage the ratchet teeth and step said ratchet wheel during movement of said plunger in said one direction between said limiting positions,

said plunger normally occupies one of said limiting positions and is movable in said one direction through a ratchet wheel driving stroke from said normal position to the other limiting position and a return stroke back to said normal position,

said plunger has a shoulder facing in the direction of said driving stroke and engaging said pawl mounting flange for locating said pawl in extended position when said mounting flange seats flat against said shoulder,

a spring on said plunger for yieldably urging said pawl mounting flange flat against said plunger shoulder to yieldably retain said pawl in extended position through said plunger driving and return strokes, and

said support has a shoulder for engaging said pawl mounting flange to rotate the pawl to retracted position against the bias of said spring when said plunger is in normal position.

4. A ratchet mechanism according to claim 3 wherein:

said spring is a coil spring surrounding said plunger and seating at one end against said pawl mounting flange and at the other end against said support for yieldably urging said flange against said plunger shoulder and yieldably urging said plunger to normal position.

5. A ratchet mechanism according to claim 4 including:

indexing recesses about said hub in axially spaced relation to said ratchet teeth, and

a resiliently yieldable indexing member mounted on said support and having an indexing detent projecting into said ratchet wheel cavity through the open side of the cavity into indexing engagement with said indexing recesses to yieldably retain said ratchet wheel in its successive positions.

6. A ratchet mechanism according to claim 5 including:

indicia about said ratchet wheel rim wall representing the wheel positions, and

means on said support cooperating with said indicia to indicate the current ratchet wheel position.

7. A ratchet mechanism according to claim 1 including:

indexing recesses about said hub in axially spaced relation to said ratchet teeth, and

a resiliently yieldable indexing member mounted on said support and having an indexing detent projecting into said ratchet wheel cavity through the open side of the cavity into indexing engagement with said indexing recesses to yieldably retain said ratchet wheel in its successive positions.

8. A ratchet mechanism according to claim 1, wherein:

said plunger is located in the plane of said ratchet wheel,

said pawl projects from said plunger along said opposite wheel side and then into said wheel cavity,

a second plunger parallel to said first plunger and located in the plane of said ratchet wheel diametrically opposite first plunger, and

a second ratchet pawl projecting from said second plunger along said opposite side of said ratchet wheel and then into said wheel cavity for driving engagement with said ratchet teeth to step said wheel in the opposite direction through said wheel positions upon successive strokes of said second plunger in one direction.

9. A ratchet mechanism according to claim 8 wherein:

said second pawl has a mounting flange with an opening loosely receiving said second plunger to rockably support the second pawl on the second plunger for rocking motion between a retracted position wherein the second pawl is disposed to clear said ratchet teeth and an extended position wherein the second pawl is disposed to engage the ratchet teeth and step said ratchet wheel during movement of said second plunger in said one direction between limiting positions,

said second plunger normally occupies one of its limiting positions and is movable in said one direction through a ratchet wheel driving stroke from its normal position to its other limiting position and a return stroke back to its normal position,

said second plunger has a second shoulder facing in the direction of its driving stroke and engaging said second pawl mounting flange for locating said second pawl in extended position when said latter mounting flange seats flat against said second shoulder,

a second spring on said second plunger for yieldably urging said second pawl mounting flange flat against said second plunger shoulder to yieldably retain said second pawl in extended position through said second plunger driving and return strokes, and

said support has a second shoulder for engaging said second pawl mounting flange to rotate the second pawl to retracted position against the bias of said second spring when said second plunger is in normal position.

10. A ratchet mechanism according to claim 9 wherein:

said springs comprise coil springs surrounding said plungers and seating at one end against the corresponding pawl mounting flanges and at the other end against said support for urging said flanges against said plunger shoulders and said plungers to normal position.

11. A ratchet assembly for use in a ratchet mechanism,

comprising:

a plunger having a shoulder facing one end of the plunger,

a ratchet pawl having a mounting flange with an opening loosely receiving said plunger, a midportion in a plane parallel to and laterally spaced from the plunger, and a tip normal to said plane and projecting over the plunger, and

a coil spring on said plunger for yieldably urging said mounting flange against said plunger shoulder. 

1. A ratchet mechanism comprising: a support; a hollow ratchet wheel rotatably mounted on said support and including a side wall, a coaxial hub projecting axially from one side of said side wall at the center of the wall and having ratchet teeth about its circumference, a coaxial cylindrical rim wall projecting axially from said one side of said side wall along the perimeter of the side wall and in surrounding relation to said hub, and said rim wall being radially spaced from and having at least substantially the same axial extent as said hub to define with the hub an annular cavity opening axially through the side of the ratchet wheel opposite said side wall; a plunger mounted on said support for longitudinal movement along an axis parallel to the plane of the wheel between two limiting positions; means for moving said plunger between said limiting positions; and a ratchet pawl carried by said plunger and projecting from said plunger into said ratchet wheel cavity through the open side of the cavity in a direction transverse to both said plunger and plane for driving engagement with said ratchet teeth to step the ratchet in one direction through successive positions upon successive strokes of said plunger in one direction.
 2. A ratchet mechanism according to claim 1, wherein: said plunger is located in the plane of said ratchet wheel, and said pawl projects from said plunger along said opposite wheel side and then into said wheel cavity.
 3. A ratchet mechanism according to claim 2, wherein: said pawl has a mounting flange with an opening loosely receiving said plunger to rockably support the pawl on the plunger for rocking motion between a retracted position wherein the pawl is disposed to clear said ratchet teeth and an extended position wherein the pawl is disposed to engage the ratchet teeth and step said ratchet wheel during movement of said plunger in said one direction between said limiting positions, said plunger normally occupies one of said limiting positions and is movable in said one direction through a ratchet wheel driving stroke from said normal position to the other limiting position and a return stroke back to said normal position, said plunger has a shoulder facing in the direction of said driving stroke and engaging said pawl mounting flange for locating said pawl in extended position when said mounting flange seats flat against said shoulder, a spring on said plunger for yieldably urging said pawl mounting flange flat against said plunger shoulder to yieldably retain said pawl in extended position through said plunger driving and return strokes, and said support has a shoulder for engaging said pawl mounting flange to roTate the pawl to retracted position against the bias of said spring when said plunger is in normal position.
 4. A ratchet mechanism according to claim 3 wherein: said spring is a coil spring surrounding said plunger and seating at one end against said pawl mounting flange and at the other end against said support for yieldably urging said flange against said plunger shoulder and yieldably urging said plunger to normal position.
 5. A ratchet mechanism according to claim 4 including: indexing recesses about said hub in axially spaced relation to said ratchet teeth, and a resiliently yieldable indexing member mounted on said support and having an indexing detent projecting into said ratchet wheel cavity through the open side of the cavity into indexing engagement with said indexing recesses to yieldably retain said ratchet wheel in its successive positions.
 6. A ratchet mechanism according to claim 5 including: indicia about said ratchet wheel rim wall representing the wheel positions, and means on said support cooperating with said indicia to indicate the current ratchet wheel position.
 7. A ratchet mechanism according to claim 1 including: indexing recesses about said hub in axially spaced relation to said ratchet teeth, and a resiliently yieldable indexing member mounted on said support and having an indexing detent projecting into said ratchet wheel cavity through the open side of the cavity into indexing engagement with said indexing recesses to yieldably retain said ratchet wheel in its successive positions.
 8. A ratchet mechanism according to claim 1, wherein: said plunger is located in the plane of said ratchet wheel, said pawl projects from said plunger along said opposite wheel side and then into said wheel cavity, a second plunger parallel to said first plunger and located in the plane of said ratchet wheel diametrically opposite first plunger, and a second ratchet pawl projecting from said second plunger along said opposite side of said ratchet wheel and then into said wheel cavity for driving engagement with said ratchet teeth to step said wheel in the opposite direction through said wheel positions upon successive strokes of said second plunger in one direction.
 9. A ratchet mechanism according to claim 8 wherein: said second pawl has a mounting flange with an opening loosely receiving said second plunger to rockably support the second pawl on the second plunger for rocking motion between a retracted position wherein the second pawl is disposed to clear said ratchet teeth and an extended position wherein the second pawl is disposed to engage the ratchet teeth and step said ratchet wheel during movement of said second plunger in said one direction between limiting positions, said second plunger normally occupies one of its limiting positions and is movable in said one direction through a ratchet wheel driving stroke from its normal position to its other limiting position and a return stroke back to its normal position, said second plunger has a second shoulder facing in the direction of its driving stroke and engaging said second pawl mounting flange for locating said second pawl in extended position when said latter mounting flange seats flat against said second shoulder, a second spring on said second plunger for yieldably urging said second pawl mounting flange flat against said second plunger shoulder to yieldably retain said second pawl in extended position through said second plunger driving and return strokes, and said support has a second shoulder for engaging said second pawl mounting flange to rotate the second pawl to retracted position against the bias of said second spring when said second plunger is in normal position.
 10. A ratchet mechanism according to claim 9 wherein: said springs comprise coil springs surrounding said plungers and seating at one end against the corresponding pawl mounting flanges and at the other end against said suppoRt for urging said flanges against said plunger shoulders and said plungers to normal position.
 11. A ratchet assembly for use in a ratchet mechanism, comprising: a plunger having a shoulder facing one end of the plunger, a ratchet pawl having a mounting flange with an opening loosely receiving said plunger, a midportion in a plane parallel to and laterally spaced from the plunger, and a tip normal to said plane and projecting over the plunger, and a coil spring on said plunger for yieldably urging said mounting flange against said plunger shoulder. 